1. Field
This invention relates to oscillators, and more specifically, to an RC oscillator based on a delay-free comparator.
2. Background
The resistive-capacitive (RC) oscillator is an important block in very-large-scale integration (VLSI) systems. It provides a reference clock when a crystal oscillator is not available or off for power saving purpose. However, the frequency accuracy of the RC oscillator is limited by RC process variation, resistor temperature coefficient, and comparator delay variation.
The temperature variation of the comparator delay is the major contributor of the frequency variation. Conventionally, the bias current of the comparator is boosted up to make the total delay a small fraction of the clock period. However, the comparator delay saturates at high bias current. To counter this effect, the device size is increased. However, this results in even more current requirement. Therefore, the power efficiency of the RC oscillator prevents it from use in low power applications. Other solutions include selecting low frequency for sleep-mode clock to bypass the delay issue or applying a closed-loop integrator to force the comparator trip voltage to be equal to the reference.